There is a RPG system here that has a monster called Borbarad-Mosquitos (named after the dark mage who created them) they do not suck blood but memories (aka experience points) you can imagine how much players love them or why a fly swatter +3 suddenly sounds like a legendary weapon...

Reminds me of a certain online retro rpg that has a combat system that contains a drain attacktype. When you get hit by that, you lose a certain percentage of your XP. If your XP falls below the requirement for your current level, you begin to lose levels. Very, very nasty. As you can guess, rings that grant 100% drain resistance are in high demand. :P Even "common" monsters like vampire feature drain attacks, so the usual reaction upon encountering one is "run like hell!"

There's also a death attacktype: if the attacker has a higher level than the victim, then the victim has a chance of simply dropping dead on the spot. There's a grimreaper monster that sports this attacktype. This sounds a lot worse, but due to poor game balancing, it's actually not so bad in practice since you can just keep your distance and take out the monster with ranged attacks, and when you get high level enough death resistance rings become common. And once you pass a certain level, you practically become immune because the monster's level will generally be lower than yours.